The heat transfer deterioration (HTD) of supercritical water in heated vertical tubes at high heat flux to mass flow rate ratios is investigated using direct numerical simulations at an inlet Reynolds number of ${Re}_{b0}=5400$ based on the inlet bulk velocity and tube diameter. The heated tube has a length of 75 times the tube diameter. Both forced and mixed convections (upward and downward flows) are simulated. The results show that primary and secondary HTDs occur in all flows considered herein. The causes of the HTD are comprehensively analyzed using the Fukagata–Iwamoto–Kasagi identity, turbulent heat flux, turbulence production, and turbulent kinetic energy. The FIK decomposition shows that the turbulent contribution ${Nu}_2$ is the dominant part of the total Nusselt number ${Nu}_{\mathit{FIK}}$ . The turbulence reduction caused by flow acceleration is the main reason for the decrease in ${Nu}_2$ and the occurrence of the primary HTD. Furthermore, buoyancy first damps the turbulence, exacerbating the HTD, and then forms an M-shaped velocity profile, which enhances the heat transfer. The secondary HTD, which is less pronounced than the primary one, comes from the decrease in the mean enthalpy gradient and enthalpy fluctuation caused by the position variation of the maximum specific heat.